1. Field of the Invention
The present invention relates to a wireless communication apparatus and method used for a wireless communication system such as a wireless LAN. In particular, the present invention relates to a method of detecting the header of a frame.
2. Description of the Related Art
According to a wireless LAN system conforming to the IEEE 802.11 standard, a wireless signal is transmitted in a state of being divided into independent frames. A receiver has a need to wait in a receivable state due to frames that may arrive at an unexpected timing. The header of a frame must be detected to receive the frame that has arrived. Conventionally, for example, there has been proposed the following method. According to this method, two average values of signal power at two different durations are determined. Judgment of presence of a header of a frame is determined based on when the difference between two average values exceeds a predetermined reference value. For example, U.S. Pat. No. 7,116,942 discloses the following technique. According to the technique, the header of a received signal is detected using two methods, that is, a change of a power value of a received signal and a change of a correlation value.
A signal received by a wireless communication apparatus is affected by factors such as noise, as well as interference of the desired wave. If the foregoing apparatus receives such an influence when a frame actually arrives, the following error detection occurs. Specifically, the apparatus misses the arrival of the frame, i.e., does not detect it, and reacts to the noise and not the frame; and consequently, starts demodulation. For example, if a reference value for detecting the header of a frame is reduced in order to avoid missing, the possibility of reacting to noise becomes high; as a result, many error detections occur. Conversely, if the reference value is increased to avoid the error detection, there is a high possibility that the header of a frame to be inherently received is not detected; as a result, missing increases. The foregoing missing and error detection are inevitable problems whenever a signal containing noise and an interference signal is received. The foregoing problem is not solved even if the technique disclosed in U.S. Pat. No. 7,116,942 is employed. As described above, conventionally, there is a problem that frame reception is not effectively performed due to missing of a received signal and error detection; therefore, improvement is desired.